Molded magnetic article

ABSTRACT

A molded magnetic article and fabrication method are provided. Particles of ferromagnetic material embedded in a polymer binder are molded under heat and pressure into a geometric shape. Each particle is an oblate spheroid having a radius-to-thickness aspect ratio approximately in the range of 15-30. Each oblate spheroid has flattened poles that are substantially in perpendicular alignment to a direction of the molding pressure throughout the geometric shape.

The invention was jointly made by employees of the United StatesGovernment and contract employees during the performance of work underNASA Contract No. NAS-1-20045. In accordance with 35 U.S.C. 202, thecontractor elected not to retain title.

ORIGIN OF THE INVENTION

Pursuant to 35 U.S.C. §119, the benefit of priority from provisionalapplication Ser. No. 60/015,154, with a filing date of Apr. 10, 1996, isclaimed for this non-provisional application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to molded magnetic materials. More specifically,the invention is a molded magnetic article such as a transformer coreand a fabrication method therefor.

2. Description of the Related Art

Power transformers typically make use of magnetic cores made from softmagnetic materials, i.e., materials having a magnetization directionthat can be easily changed. Geometric shapes for such cores generallyinclude complex geometric shapes that define a closed path, e.g., ahollow square or rectangular "picture frame" shape, a donut shape, etc.The magnetic cores are generally fabricated from magnetic ingots thatmold-set into a desired shape using heat and pressure. Current magneticcore fabrication methods require extremely high pressures (e.g., on theorder of 250 kilograms per square inch (ksi) in order to achieveacceptable levels of magnetic induction saturation or B_(MAX) ofapproximately 13 kilogauss (kG). Such results are best achieved asreported by Speed et al, in "Magnetic Properties of Compressed PowderedIron", Transaction of American Institute of Electrical Engineers, VolumeXL, p. 1321-1359, 1921. However, the application of such high pressuresreduces the life of the mold sets thereby raising the production cost ofmagnetic cores.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amolded magnetic article and fabrication method therefor.

Another object of the present invention is to provide a molded magneticarticle and fabrication method that uses relatively low moldingpressures.

Still another object of the present invention is to provide a moldedmagnetic article having improved mechanical properties.

Yet another object of the present invention is to provide a fabricationmethod to produce magnetically soft and mechanically strong magneticcores for use in transformers.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a molded magnetic articlecomprises a plurality of particles of ferromagnetic material embedded ina polymer binder. The particles and polymer binder are molded underpressure into a geometric shape. Each particle is an oblate spheroidhaving a radius-to-thickness aspect ratio approximately in the range of15-30. Each oblate spheroid has flattened poles that are substantiallyin perpendicular alignment to a direction of the molding pressurethroughout the geometric shape. In the method of fabrication, a molddefines the geometric shape of the article. The mold is filled with theparticles and polymer binder which are heated to a temperature thatcauses the polymer binder to flow. Pressure is applied to the mixture inthe mold in a direction that is perpendicular to a desired axis ofmagnetization of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the fabrication process of thepresent invention used to form a rod-shaped molded magnet;

FIG. 2 is a side view of one oblate spheroid shaped particle fordescribing the aspect ratio thereof in the present invention;

FIG. 3 is a cross-sectional view of one oblate spheroid shaped particlecoated with an electrically insulating polymer binder for use in thefabrication of a soft magnet in accordance with the present invention;

FIG. 4 is a schematic representation of the fabrication process thatfurther includes the step of applying a magnetic field to help align theparticles along the desired direction of magnetization of the moldedarticle;

FIG. 5A is a planar view of one embodiment of a "picture frame" shapedtransformer core in accordance with the present invention;

FIG. 5B is a planar view of one embodiment of a donut-shaped transformercore in accordance with the present invention;

FIG. 6A is a planar view of an alternative embodiment "picture frame"shaped transformer core; and

FIG. 6B is a planar view of an alternative embodiment donut shapedtransformer core.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, aschematic representation of a fabrication process according to thepresent invention is shown. By way of example, FIG. 1 will be used to 10describe the fabrication of a simple molded rod 10 which can bemagnetized as either a hard (i.e., permanent) or soft (ie., changeabledirection of magnetization) magnet. However, the fabrication process andultimately formed article resulting therefrom can assume other geometricshapes such as the variety of closed-loop shapes (e.g., a hollow squareor rectangular "picture frame" shape, a donut shape, a toroid, etc.)generally used as power transformer cores.

Regardless of shape and/or the hard or soft nature of magnetic articlebeing formed, a mold 12 is provided and is filled with particles 14 offerromagnetic material (e.g., cobalt, iron, nickel, etc.) and a polymerbinder 16. The mixture of particles 14 and polymer binder 16 can becreated by coating each of particles 14 with polymer binder 16 as willdescribed further below. Another option is to use uncoated particles 14mixed with polymer binder 16 provided the volume percentage of polymerbinder 16 is approximately 50% or greater.

Each of particles 14 is a microscopic-sized particle having a diameteron the order of 100 microns. Each of particles 14 is shaped as an oblatespheroid as shown in the enlargement of one particle 14 in FIG. 2. Bydefinition, an oblate spheroid is defined as having flattened poles. InFIG. 2, the flattened poles are depicted at flat and parallel surfaces14A and 14B. In terms of the present invention, each oblate spheroid 14has an aspect ratio in the range of approximately 15-30 where the aspectratio is defined as the largest radius R_(MAX) of spheroid 14 relativethe thickness t of spheroid 14. Ferromagnetic particles meeting thesephysical criteria are commercially available from, for example, AlfaAesar, Ward Hill, Mass.

Mold 12 is filled with particles 14 and polymer binder 16 and heated.The applied heat is indicated in the drawings by arrowed wavy lines 18.Heat 18 is applied in sufficient quantity to cause polymer binder toundergo a melt or viscous flow. Accordingly, the particular temperaturerequired in the present invention will vary depending on the polymerbinder used. As polymer binder 16 begins to flow, pressure (indicated byarrows 20) is applied to particles 14 and polymer binder 16 via mold 12.Pressure 20 is applied uniaxially in a direction that is perpendicularto a desired direction of magnetization of rod 10. In other words, forthe example shown in FIG. 1, it is assumed that the desired direction ofmagnetization is along the longitudinal axis 100 thereof.

The externally applied pressure 20 aligns oblate spheroids 14 such thatall flattened poles (e.g., 14A and 14B) are substantially parallel forall spheroids in the case of a straight rod 10. In general, theflattened poles are substantially perpendicular to the direction ofapplied pressure 20 experienced within each section of mold 12. This isbecause applied pressure 20 produces torque which tends to rotate eachparticle 14 so that faces 14A and 14B tend to align perpendicular to thedirection of pressure 20 in each local volume of mold 12. Further, eachparticle 14 has its easy or preferred axis of magnetization parallel toits flattened poles 14A and 14B. This is because the relatively largeaspect ratio used in the present invention minimizes the effects thatany demagnetization field can have parallel to the flattened poles. Inthis way, the present invention causes the individual preferred axis ofmagnetization of particles 14 to be aligned with the desired axis ofmagnetization of rod 10, i.e., along the longitudinal axis thereof.

If rod 10 is to be used as a soft magnet and particles 14 areelectrically conductive, it is desirable to prevent mechanical contactbetween each of particles 14. One way of achieving this is to coat eachparticle with the electrically insulating polymer binder 16 prior tofilling mold 12. A so-coated particle 14 is shown in cross-section inFIG. 3. There are several methods that can be employed to coatelectrically conductive particles with an electrically insulatingmaterial. One such method and coating is described in co-pending U.S.patent application entitled, "Tough, Soluble, Aromatic, ThermoplasticCopolyimides", Ser. No. 08/359,752, filed Dec. 16, 1994, and having thesame assignee as the instant application, the contents of which arehereby incorporated by reference. Briefly, the aromatic soluble imidedescribed in the afore-mentioned patent application is a thermoplasticcopolyimide prepared by reacting 4,4'-oxydiphthalic anhydride with3,4,3',4'-biphenyltetracarboxylic dianhydride and 3,4'-oxydianiline. Useof this aromatic soluble imide is particularly advantageous because oncethe drying temperature is above the coating softening point, the coatingbecomes insoluble while remaining thermoplastic. Thus, multiple coats ofthe polymer can be applied to particles 14 without removing a previouscoating layer prior to consolidation. The multiple-coating capabilityprovides the necessary assurance that all particles 14 are adequatelycoated with polymer binder 16. Further, if necessary, thick coatings ofpolymer binder 16 can be applied to particles 14.

The molded magnetic article and fabrication method of the presentinvention could also make use of other high-performance polymer bindersfor metals (particularly iron) as disclosed in U.S. Pat. Nos. 5,063,011,5,198,137 and 5,225,459.

These patents outline several procedures for coating metallic particleswith thermoplastics and pressing them to form tough green metallicingots which are particularly useful in magnetic components.

If it is found that pressure 20 alone does not produce sufficientalignment of particles 14 as described above (i.e., the magnet strengthalong the desired axis of magnetization of rod 10 is not as great asexpected), the fabrication method of the present invention can includethe step of applying a magnetic field to help properly align particles14. This situation is depicted in FIG. 4 where a magnetic field(represented by arrow 22) is applied or induced in rod 10 in a directionthat is parallel to the desired direction of magnetization of rod 10.Accordingly, in the illustrated example, magnetic field 22 is parallelto longitudinal axis 100 of rod 10. The application of magnetic field 22can occur prior to or during the application of heat 18 and pressure 20.

As mentioned above, the fabrication process of the present invention canbe used to make soft-magnet transformers cores that involve a variety ofclosed-loop shapes. Several such transformer cores are depicted in FIGS.5A, 5B, 6A and 6B. In the embodiments in FIGS. 5A and 5B, a "pictureframe" shaped core 30 and donut shaped core 40 are depicted,respectively. If the desired direction of magnetization is in the planeof the paper, the direction of applied pressure is perpendicular to theplane of the paper. This will align particles 14 such that theirflattened poles are parallel to the plane of the paper throughout theentire geometric shape. If necessary, applied magnetic field 22 could beinduced about the closed-loop path defined by each of cores 30 and 40 inorder to help with alignment of particles 14. In the embodimentsdepicted in FIGS. 6A and 6B, it is assumed that the desired direction ofmagnetization is perpendicular to the plane of the paper. In FIGS. 6Aand 6B, applied pressure 20 causes particles 14 to be aligned such thattheir flattened poles (e.g., faces 14A and 14B) are perpendicular to theplane of the paper throughout the entire geometric shape.

The molded magnetic articles fabricated in accordance with the presentinvention exhibit significant resistance to bending moments appliedperpendicular to the aligned flattened poles of the particles. This isespecially true when the particles are made from a rigid ferromagneticmaterial, e.g., iron, since each oblate spheroid resists bending momentsperpendicular to the flattened poles thereof.

The advantages of the present invention are numerous. Acceptable levelsof magnetic saturation having been achieved for a variety of moldedmagnetic articles utilizing molding pressures of less than 20 ksi. Suchreduced molding pressures will increase the life of mold sets andtherefore reduce the overall cost of molded magnetic articlemanufacture. The fabrication method can be used to make hard or softmagnets in a variety of simple or complex geometries. The fabricationprocess is well-suited to the manufacture of soft-magnet transformercores. The process is simple and can be implemented with current moldingequipment.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A molded magnetic article comprising aplurality of particles of ferromagnetic material embedded in a polymerbinder, said plurality of particles embedded in said polymer binderbeing molded under pressure into a geometric shape, each of saidplurality of particles being an oblate spheroid having aradius-to-thickness aspect ratio approximately in the range of 15-30,each said oblate spheroid having flattened poles wherein said flattenedpoles of said plurality of particles are substantially in perpendicularalignment to a direction of said pressure throughout said geometricshape.
 2. A molded magnetic article as in claim 1 wherein saidferromagnetic material is an electrically conductive material.
 3. Amolded magnetic article as in claim 1 wherein said polymer binder is anaromatic soluble imide binder.
 4. A molded magnetic article as in claim3 wherein said aromatic soluble imide binder is a thermoplasticcopolyimide.
 5. A molded magnetic article as in claim 4 wherein saidthermoplastic copolyimide is generated from a reaction of4,4'-oxydiphthalic anhydride with 3,4,3',4'-biphenyltetracarboxylicdianhydride and 3,4'-oxydianiline.
 6. A molded magnetic article as inclaim 1 wherein said ferromagnetic material is selected from the groupconsisting of cobalt, iron and nickel.
 7. A molded magnetic article asin claim 1 wherein said geometric shape is a straight rod.
 8. A moldedmagnetic article as in claim 1 wherein said geometric shape defines aclosed-loop shape.
 9. A molded magnetic article as in claim 1, whereinsaid perpendicular alignment of said flattened poles of each said oblatespheroid is uniaxial compressive stress-induced, said uniaxialcompressive stress induced by pressure applied substantiallyperpendicular to said flattened poles.